| >| >What I think the Williams/Dawkins concept of adaptation requires is that,| >>if there is a selection bias for a unit of hereditary change (including| >>cultural units), *then* it must be an adaptation by definition, and what| | | One minor point, but one I think we all could agree on, is that it isn't| possible always to speak of the fitness of an allele (a unit of| hereditary change). A classic example is the hemoglobin S gene. Homozygosity| for a particular allele of that gene causes sickle cell anemia. | Heterozygosity, with one allele of the normal type, confers a fitness| benefit: resistance to malaria. Since the heterozygous condition is more| fit, we can't really speak of the fitness of individual alleles. By| William's definition, the sickle cell allele would be considered an| adaptation.

"At times it may be necessary to investigate selection in the different
genetic environments of one population for an understanding of apraticular
problem of adaptation. In very malarial parts of Africa a common gene
produces homozygotes with a fatal disease, sickle -cell anemia, and viable
heterozygotes with high resistance to malaria... Its allele produces a
homozygote that is normally viable but normally susceptible to malaria. If
the gene associated with anemia and malarial resistance is designated S,
its selection coefficient in the genetic environment S would be very
different from its coefficient in environment S'. Its effective (mean)
coefficient would be the mean of these two environments, weighted by the
frequencies of the environments. These coefficients change in time and
space as a function of the incidence of malaria." 1966:60

So according to Williams' treatement, it can be an adaptation depending
upon the relative selection coefficients of alleles, or not. That surely
makes sense. What sense of "adaptation" does not refer to alternative
traits, in order to determine the relative advantage?